Generally, secondary batteries include a nickel-cadmium battery, a nickel-hydrogen battery, a lithium ion battery, a lithium ion polymer battery and so on. Such secondary batteries are classified into lithium-based batteries and nickel-hydrogen-based batteries. The lithium-based batteries are mainly used for small products such as digital camera, P-DVD, MP3P, cellular phone, PDA, portable game device, power tool and E-bike, while the nickel-hydrogen-based batteries are mainly applied to and used for large products such as electric or hydride vehicles, which require high output.
Meanwhile, an electric or hybrid electric vehicle is operated by driving a high power motor, so a high voltage battery of 100V or above is used. Thus, a high voltage battery used for an electric or hybrid electric vehicle has a plurality of battery cells connected in series or in parallel. The high voltage battery having a plurality of battery cells uses a voltage measuring circuit to periodically measure voltage of each battery cell and thus control charging/discharging of the battery.
FIG. 1 is a circuit diagram showing a conventional battery cell voltage measuring apparatus.
As shown in FIG. 1, the conventional voltage measuring apparatus includes a first switching unit 31 composed of a plurality of switch devices respectively connected to anode and cathode terminals of each battery cell VB1, VB2, VB3, VB4 for sensing voltages at both terminals of each battery cell VB1, VB2, VB3, VB4 included in a cell group 21 of a battery 20 that is connected to a load 10 to supply power thereto, a voltage charging unit 32 charged with the voltage of each battery cell VB1, VB2, VB3, VB4 by means of turning-on operation of the first switching unit 31, and a second switching unit 33 for applying the voltages at both ends of the voltage charging unit 32 to a voltage amplifier 40 by means of its turning-on operation.
However, in the above conventional battery voltage measuring apparatus, the battery 20 is directly connected to a ground to which the load 10 is grounded, while being electrically connected to the load 10. In this connection, in order to measure voltage of each battery cell VB1, VB2, VB3, VB4, the voltage amplifier 40 and the load 10 connected through the first and second switching units 31, 33 are connected to the same ground, so a high voltage corresponding to the entire voltage of the battery is applied to the first and second switching units 31, 33. Thus, switch devices included in the first and second switching units 31, 33 should have high withstanding voltage so as to endure high potential.
A switch device with high withstanding voltage is however expensive, thereby causing increase of production costs of the voltage measuring apparatus. Thus, there is an urgent need in the art for developing a circuit capable of measuring voltage of each battery cell by adopting a device with a low withstanding voltage.